[427] The following is
a brief explanation of planetary motions and, in particular, the
relationships of the orbits of Earth and Mars. Mercury is the
planet closest to the sun. Venus, Earth, Mars, the asteroids,
Jupiter, Saturn, Neptune, and Pluto, in that order, are farther
out, All move around the sun in the same direction, If the solar
system were viewed from far above Earth's northern hemisphere, the
planetary motion would appear to be counterclockwise, The
planetary orbits lie in very nearly the same plane. The paths of
the planets, if seen from the sun, would all describe the same
circle, except for Mercury and Pluto, which have tilted orbits-
the innermost and outermost of the sun's satellites.

The planets move in the same direction and
most of them occupy a common plane, but the distance between any
two planets varies considerably with time, Figure l illustrates
the perihelia (points of closest approach to the sun) and aphelia
(farthest points from the sun) of Mercury and Mars. Earth and
Venus have more nearly circular orbits. Since Earth and Mars
travel around the sun in orbits of different lengths with
different velocities, the distance between the planets varies
constantly. About every 780 days (the actual interval ranges from
765 to 810 days), Earth overtakes its slower neighbor in their
unending orbiting of the sun. (Kepler pointed out in his third
law: The square of the period of a planet's complete revolution
around the sun is proportional to the cube of its mean distance
from the sun.) Seen from the sun, the two planets momentarily lie
along a straight line. Seen from Earth, Mars is in a...

Fig. 1. Orbits of the inner
planets of the solar system are drawn to scale (although the sizes
of the planets are not). Orbits of Venus and Earth are nearly
circular. Mercury and Mars orbits are eccentric. Perihelion points
of Mercury and Mars are indicated by [Greek letter pi] and apelion
by a (Greek letter alpha).

[428]

Fig. 2. At conjunction of Mars and
Earth, Mars becomes invisible in the sun's rays. Best observation
period is from quadrature to quadrature, Samuel Glasstone, The Book
of Mars, NASA SP-179,1968.

...direction directly opposite that of the
sun; Mars and the sun cannot be seen at the same time because they
are on opposite sides of the Earth. This positioning, illustrated
in figure 2, is called opposition. During opposition, Mars and
Earth come closest together- between 55 and 102 million
kilometers.

Distances between
the Two Planets

.

Date

Kilometers

.

30 Dec. 1960

90 606 067

4 Feb. 1963

100 101 196

9 Mar. 1965

99 779 328

15 Apr. 1967

89 801 395

31 May 1968

89 801 395

10 Aug. 1971

56 166 105

25 Oct. 1973

65 017 497

15 Dec. 1975

84 329 625

As Earth keeps racing ahead and Mars falls
behind, there are instances when the two planets form a straight
line, with the sun interposed between them. Mars disappears from
Earth's view behind the disk of the sun; the planets are in
conjunction. Mars is as far away from Earth as it can be- more
than 350 million kilometers.

One other position in the Earth-Mars
relationship is also important. When the sun, Earth, and Mars
describe a right angle, Mars is said to be in quadrature. In this
position, Mars does not appear as a round disk to Earth-based
observers. Instead, it looks like the gibbous moon, between half-
and full-moon phases. What we do not see is the night side of Mars
blending with the black sky.

If an opposition takes place along the
line marked 0° on figure I, an observer on Earth would look
across a shorter distance to Mars than during a 180°
opposition. An opposition at Mars perihelion would offer the best
opportunity for observations and spaceflight, since the distance
between Earth and Mars at that point would be the shortest. Of the
more recent oppositions, the one in 1924 came the closest to being
at perihelion, while the 1933 opposition was almost precisely at
aphelion.

For further information, see Samuel
Glasstone, The Book of Mars, NASA SP-179 (Washington, 1968), and
Willy Ley and Wernher von Braun, The Exploration of Mars (New
York: Viking Press, 1956).

[429]

Fig. 3. The orbits of Earth and Mars,
showing the times of opposition and the separation at opposition.
Samuel Glasstone, The Book of Mars, NASA SP-179, 1968.